Check valves are used in a variety of situations, and particularly where the flow of a media, such as gaseous exhaust or fluid discharge, is desired in one direction. One example where flow in only one direction is not only desired, but critical, is in the silicon wafer manufacturing processes where reactors are used to process silicon wafers. Due in part to the nature of the materials used and processes performed, these reactors exhaust gases that may be extremely hazardous and/or toxic. These gases are typically exhausted through a manifold into an exhaust stream that may undergo further treatment before final discharge.
In certain systems, there may often be a slight positive pressure in the exhaust stream, or situations may arise where a positive pressure in the exhaust stream is created. To prevent backflow of the media being exhausted, a check valve may be used between the reactor and the exhaust stream. Backflow is undesirable, as it may force hazardous or toxic material out of the system, through a reactor chamber, or contaminate a section of a reaction chamber. Such a discharge can further result in exposure to personnel and result in potential contamination of silicon wafers in process. Check valves are used in other processes as well, as backflow can have just as serious consequences.
A variety of check valves are known. One common check valve includes a simple rigid valve flap, or swing check valve, attached to an piping body in a hinged fashion such that flow of media through the opening forcing the flap of the valve to swing away from the opening. When media is not being exhausted, the flap may close over the opening through the influence of gravity and the backflow pressure to prevent backflow of the media. This type of check valve may work well in situations where the downstream to upstream pressure differential is larger (e.g., much greater than two inches of water column) or where there is a substantial negative pressure downstream that tends to pull the exhaust away from the reactor chamber.
These valves are relatively ineffective, however, where the pressure differentials are extremely low, such as, for example, less than or equal to one inch of water column. A greater amount of back pressure is required than is typically present in these systems to have the rigid flap of the check valve sufficiently seal against the orifice in order to prohibit the exhaust from flowing back into the reactor chamber. Additionally, whereas in the case of reactors in the silicon manufacturing process, where there is a slight positive pressure downstream from the valve, these valves do not react fast enough, or seal sufficiently enough, to prevent exhaust from flowing back into the reactor chamber and out to the operating environment if the reactor chamber is open to the atmosphere.
A second common type of check valve includes spring-biased valves, similar to those disclosed in U.S. Pat. Nos. 4,867,200 and 5,060,689. In these systems, a spring maintains a constant bias against an orifice plate such that only upstream pressure can displace the orifice plate against the spring allowing the exhaust to enter the exhaust stream. However, if the upstream pressure is reduced, for example becomes atmospheric, the spring forces the orifice plate against the orifice to prevent backflow. The spring-biased check valves may not operate sufficiently or effectively at extremely low pressure differentials, in that they may require too much “cracking pressure”: (i.e. greater than 2 inches of water column, which may be impermissible in certain systems). Further, the spring check valves have more parts that may be prone to failure. Moreover, the spring-biased check valves are usually more invasive in piping systems. Therefore, an improved, low pressure check valve is needed.